Implement a synchronous FIDL client in Rust
Prerequisites
This tutorial assumes that you are familiar with writing and running a Fuchsia component and with implementing a FIDL server, which are both covered in the FIDL server tutorial. For the full set of FIDL tutorials, refer to the overview.
Overview
This tutorial implements a client for a FIDL protocol and runs it against the server created in the previous tutorial. The client in this tutorial is synchronous. There is an alternate tutorial for asynchronous clients.
If you want to write the code yourself, delete the following directories:
rm -r examples/fidl/rust/client_sync/*
Create a hello world component
Set up a hello world component in
examples/fidl/rust/client_sync
. You can name the componentecho-client
, and give the package a name ofecho-rust-client-sync
.Note: If necessary, refer back to the previous tutorial.
Once you have created your component, ensure that the following works:
fx set core.x64 --with //examples/fidl/rust/client_sync
Build the Fuchsia image:
fx build
In a separate terminal, run:
fx serve
In a separate terminal, run:
fx shell run fuchsia-pkg://fuchsia.com/echo-rust-client-sync#meta/echo-client.cmx
Edit GN dependencies
Add the following dependencies to the
rustc_binary
:{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/rust/client/BUILD.gn" region_tag="deps" %}
Then, import them in
main.rs
:{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/rust/client/src/main.rs" region_tag="imports" %}
These dependencies are explained in the server tutorial.
The one new dependency is fuchsia-zircon
, which is a crate containing type safe
bindings for making Zircon kernel syscalls. In this example, the crate is used to
create a channel.
Edit component manifest
Include the
Echo
protocol in the client component’s sandbox by editing the component manifest inclient.cmx
.{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/rust/client_sync/client.cmx" %}
Connect to the server {#main}
The steps in this section explain how to add code to the main()
function
that connects the client to the server and makes requests to it.
Initialize a channel
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/rust/client_sync/src/main.rs" region_tag="main" highlight="2,3" %}
This channel will be used to communicate between the client and server.
Connect to the server
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/rust/client_sync/src/main.rs" region_tag="main" highlight="4,5,6,7,8" %}
connect_channel_to_service
will bind the provided channel end to the specified
service. Under the hood, this call triggers a sequence of events that starts on the client and traces through the server code from the previous tutorial:
- Makes a request to the component framework containing the name of the service to connect to, and the
other end of the channel. The name of the service is obtained implicitly using the
SERVICE_NAME
ofEchoMarker
template argument, similarly to how the service path is determined on the server end. - This client object is returned from
connect_to_protocol
.
In the background, the request to the component framework gets routed to the server:
- When this request is received in the server process,
it wakes up the
async::Executor
executor and tells it that theServiceFs
task can now make progress and should be run. - The
ServiceFs
wakes up, sees the request available on the startup handle of the process, and looks up the name of the requested service in the list of(service_name, service_startup_func)
provided through calls toadd_service
,add_fidl_service
, etc. If a matchingservice_name
exists, it callsservice_startup_func
with the provided channel to connect to the new service. IncomingService::Echo
is called with aRequestStream
(typed-channel) of theEcho
FIDL protocol that is registered withadd_fidl_service
. The incoming request channel is stored inIncomingService::Echo
and is added to the stream of incoming requests.for_each_concurrent
consumes theServiceFs
into a [Stream
] of typeIncomingService
. A handler is run for each entry in the stream, which matches over the incoming requests and dispatches to therun_echo_server
. The resulting futures from each call torun_echo_server
are run concurrently when theServiceFs
stream isawait
ed.- When a request is sent on the channel, the channel the
Echo
service is becomes readable, which wakes up the asynchronous code in the body ofrun_echo_server
.
Note: If any requests are sent before the server end of the channel is bound to the server, these requests are buffered by the kernel. The server then reads these requests as soon as the server is initialized.
Send requests to the server
The code makes two requests to the server:
- An
EchoString
request - A
SendString
request
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/rust/client/src/main.rs" region_tag="main" highlight="10,11,12,13,14,15" %}
The call to echo_string
will block until a response is received from the server, and therefore
it takes a timeout argument as the last parameter.
On the other hand, the call to send_string
does not have a timeout parameter since SendString
does not have a response. With the current server implementation, an OnString
event
will be sent to the client after this request is received. However, the synchronous Rust bindings
do not have support for handling events.
The [bindings reference][bindings-ref] describes how these methods are generated, and the [Fuchsia rustdoc][rustdoc] includes documentation for the generated FIDL crates.
Run the client
If you run the client directly, it will not connect to the server correctly because the
client does not automatically get the Echo
protocol provided in its
sandbox (in /svc
). To get this to work, a launcher tool is provided
that launches the server, creates a new Environment
for
the client that provides the server’s protocol, then launches the client in it.
Configure your GN build:
fx set core.x64 --with //examples/fidl/rust/server --with
//examples/fidl/rust/client_sync --with //examples/fidl/test:echo-launcher
Build the Fuchsia image:
fx build
Run the launcher by passing it the client URL, the server URL, and the protocol that the server provides to the client:
fx shell run fuchsia-pkg://fuchsia.com/echo-launcher#meta/launcher.cmx fuchsia-pkg://fuchsia.com/echo-rust-client-sync#meta/echo-client.cmx fuchsia-pkg://fuchsia.com/echo-rust-server#meta/echo-server.cmx fuchsia.examples.Echo
You should see the print output in the QEMU console (or using fx log
).
[109100.005] 505615:505617> Listening for incoming connections...
[109100.099] 505615:505617> Received EchoString request for string "hello"
[109100.100] 505615:505617> Response sent successfully
[109100.100] 505864:505866> response: "hello"
[109100.100] 505615:505617> Received SendString request for string "hi"
[109100.100] 505615:505617> error sending event
[109100.100] 505615:505617>
[109100.100] 505615:505617> Caused by:
[109100.100] 505615:505617> 0: A server encountered an IO error writing a FIDL response to a channel: PEER_CLOSED
[109100.100] 505615:505617> 1: PEER_CLOSED
The server prints a PEER_CLOSED
error when it tries to send the event, because the client
terminates immediately after sending the SendString
request without waiting for the OnString
event. This is expected since synchronous clients cannot handle events. To see how to handle
events, try following the async client tutorial instead